Having just discovered this thread, I read through it and if the following info was mentioned, I didn't catch it.
Your rifle was definitely built as a gallery model and the mismatched serial numbers is almost positive proof that it spent some time in a working shooting gallery. A gallery model has a distinct loading port cut into the magazine tube as opposed to a non-gallery model.
First of all, all gallery models were chambered in .22 short. I don't know when it started but both Remington and Winchester-Western manufactured special gallery ammunition called "spatterless" gallery ammo (there is no "l" in the word). The projectiles were not cast, but were made from pressed lead powder. Hence when they hit a metal target or the backdrop of the gallery, they would disintegrate into lead dust.
(nobody was concerned about breathing lead dust at the time)
The loading port for a standard model was simply a cutout shaped like the profile of a .22 cartridge-only larger. The loading port on a gallery model is shaped like a large triangular keyhole just like yours. This is because galleries didn't reload the guns by hand--one bullet at a time. Instead they used speed load tubes, which Winchester provided (at a price of course) These were nothing more than metal tubes closed on one end that would hold up to 20 of the shorts. These were kept in racks with the open end of the tube up. When a gun needed to be reloaded, the gallery worker pulled the inner spring tube to clear the loading port. and then would hold his finger over the open end of the speed load tube until he could insert it into the keyhole shaped port. At that point, gravity took over and the magazine was loaded instantaneously. Then while customers were shooting, the gallery workers had time to reload the speed tubes the old fashioned way.
Lastly, as mentioned, your upper and lower receiver sections having mismatched serial numbers--is an almost sure sign that your rifle spent at least some time as a working gallery gun. This comes from the fact that when the gallery workers got around to cleaning the guns, they did it during down time and did all the guns at once. They simply didn't bother to match up the serial numbers when they reassembled the two halves of the gun.
Nice restore by the way.
P.S.
Here's a rare box of the spatterless gallery ammo for sale currently on GB.com
https://www.gunbroker.com/item/776163616
Cheers